Epitaxial graphene growth is often accompanied by step bunching of the underlying SiC substrate and graphene bilayer formation which can deteriorate the quality of graphene-based devices, e.g., the resistance quantization of the quantum Hall effect. We show AFM, STM, Raman and electronic transport data which indicate that improved buffer layer growth is the key to obtain homogenous large-area monolayer graphene. Particularly, the substantial impact of the so-far less regarded Ar flow rate on the graphene quality is investigated in this study and explained by a quasi-equilibrium model at the growing surface. The quality of our ultra-smooth graphene layers is proven by the high uniformity of quasi-freestanding graphene sheets obtained by hydrogen intercalation which is underlined by the very small resistance anisotropy of such samples on um and mm scales.